Alkaline cells and methods of storing them



United States Patent ALKALINE CELLS AND METHODS OF STORING THEM HowardJ. Strauss, Philadelphia, Pa., assignor to The Electric Storage BatteryCompany, a corporation of New Jersey No Drawing. Application February17, 1955 Serial No. 488,985 7 1 Claim. (Cl. 136-161) This inventionrelates to electric batteries of the hermetically sealed type and hasfor an object the provision of a method of and means for producing andmaintaining electric batteries at full charge with a minimum ofmaintenance being required, each battery being available for instant useover long periods of time.

This invention is particularly applicable to batteries of the alkalinetype, particularly those including silver electrodes or positive platesand Zinc electrodes or negative plates. It is well known thatsilver-zinc cells have in the past had a relatively short shelf-life. Ifa cell retains 80% or" its charge over a 30-day period, it hasheretofore been considered satisfactory. To extend shelflife, it hasheretofore been proposed (Vinal, Primary Batteries, third edition, 1950,page 273) to provide between the electrodes a tight packing of asbestosfiber. A longer resultant shelf-life is obtained but at the expense of aloss in the capacity of the cell for discharge at high current rates anda loss in volumetric performance.

It is a further object of the present invention to maintain silver-zinccells at full charge over extended periods of time without loss ofcharge and without impairment of current discharge capabilities.

In carrying out the invention in one form thereof, the silver-zinc cellsor batteries are hermetically sealed, as for example, in the mannerdisclosed and claimed by me in my copending application Serial No.460,679, filed October 6, 1954. More particularly, each cell of thebattery is initially purged by oxygen which is introduced into the cellfrom a suitable source. Each cell is provided with a vent. After flow ofoxygen for a short period of time, the cell is sealed. In this manner,atmospheric gases other than oxygen are reduced to a low value, with theresult that the gas pressure within the cell approximates, issubstantially equal to, the partial pressure of oxygen within the cell.After formation of the plates to provide a full charge of the positiveplates, a floating current of a low order of magnitude is passed throughthe cells in the charging direction. It has been found that theapplication of such a floating current pro duces new and unexpectedresults in maintaining the health of the cell over an extended period oftime. After an extended period of time, at least thirty days, cells havebeen discharged Without significant loss of capacity either in terms ofinitial high discharge rate or in volumetric performance. Where thecharging rate for such cells might range from 2 to 20 amperes, thefloating current need be but of the order of 150 milliamperes. Floatingcurrents, smaller and larger than the stated figure, can be and havebeen used, it being understood that the magnitude of the floatingcurrent is not critical. Ordinarily, the lower current values will beselected for the purpose already set forth and hereinafter to be morefully explained.

All of the reasons for loss of capacity of the cell are not known, andit is doubted that all of the mechanisms involved have been identified.Nevertheless, the experimental work which I have done, taken inconjunction 2,862,989 Patented Dec. 2, 1958 ice with the resultsachieved, lead me to believe that the loss in capacity heretoforeexisting has in part been due to loss in the negative capacity of thecell. Apparently at the negative plates there is conversion of zinc tozinc oxide. When a cell is not hermetically sealed, there is not onlyabsorption from the atmosphere of carbon dioxide which increases theelectrical resistance of the cell, but also there is some absorption ofoxygen which contributes to the oxidation of the zinc.

Further in accordance with the present invention, whenever overchargeconditions prevail at the positive plates with resultant production ofoxygen, provisions are made favorable for the following reaction tooccur at the negative plates:

O +H O+4 electrons 401-1- 1) Conditions within the cell or battery arealso established which are unfavorable to the following reaction:

The use of a floating current also minimizes the possibility of shortcircuits arising from Zinc trees. If zinc is permitted to form zincoxide (as in Reaction 2) and is permitted to plate out at the zincelectrode in the form of protrusions or trees, they may extend intoelectrical contact with the positive electrodes to short-circuit thecell. If the zinc oxide remains in the cell for any length of time, itappears to concentrate at particular locations, as at the lower portionof the cell. By maintaining the floating current, there is prevented thedissolution of zinc in the electrolyte in the form of zinc oxide andalso the accumulation of zinc at the lower portion of the cell. Thus,the deleterious effects of treeing are not realized since the zincgrowths are to large degree inhibited.

Further in connection with the sealing of the cell, it is desirable toprovide an oxygen-saturated electrolyte and an oxygen atmosphere Withinthe sealed battery. In this manner the partial pressure of gases otherthan oxygen within the cell is reduced to an exceedingly low value,approaching zero as a limit. Since the gas pressure within the sealedcell will then approximate the partial pressure of oxygen, there areprovided conditions for maxi mum solubility of the oxygen in theelectrolyte and a minimum gas pressure within the cell. Moreparticularly, the fraction of gas pressure within the cell due solely tooxygen may be less than atmospheric pressure; it may be as low orlowervthan a fifth of the atmospheric pressure. It is only importantthat the partial pressure of oxygen be high as compared with the partialpressure of other gases within the cell. My experimental work indicatesthat a cell may be operated at subatmospheric pressure if gases otherthan oxygen are excluded from the cell at the time of sealing.

Further in accordance with the invention, there are desirable physicalarrangements of the parts. A close spacing between the positive platesand the negative plates is provided.

For the primary silver-zinc cell a porous and absorptive separator isinserted between the positive and negative plates with a part of theseparator protruding above the electrodes and the electrodes themselvesprotruding above the electrolyte.

For a secondary cell the diaphragm or separator, of regeneratedcellulose, generally has a low rate of gas transfer therethrough.Accordingly, in secondary cells it Will be highly desirable to providebetween the portions of the electrodes protruding above the electrolytelevel a porous, gas-permeable membrane, such as absorptive papers orgas-permeable filters. During recharging, the electrolyte level ismaintained below the porous permeable membrane to prevent shortcircuits.

What is claimed is:

In the storage of sealed electric batteries of the silverzinc type in acharged'wet condition, the method which comprises introducing oxygeninto the container prior to sealing the same to increase the partialpressure of oxygen References Cited in the file of this patent UNITEDSTATES PATENTS Tichenor Dec. 11, 1951 Andre Apr. 29, 1952 OTHERREFERENCES Vinal: Storage Batteries, third edition, 1940, pages 250 and251.

